Automated friction clutches of the above type are mainly formed as dry clutches with at least one drive disk, with contact being maintained by the spring pressure of at least one contact pressure spring, since this type of structure is particularly well developed and robust and since its axial structural length is short. In the rest condition when no actuating force is applied, its operating status is engaged, this clutch structure also enables the motor vehicle concerned to be parked safely by engaging a gear in addition to putting on the parking brake.
The friction clutch is, in each case, disengaged mainly in order to bridge speed differences between the drive engine of the motor vehicle and the input shaft of the transmission when starting off and after gear shifts, as well as for carrying out gear shifts not under load, and is then engaged in a controlled manner, such that the respective clutch torque, i.e., the torque that can be transmitted momentarily by the friction clutch (clutch capacity), is determined by a hydraulically or pneumatically actuated clutch actuator.
The clutch actuator can be made as a pressure-medium-actuated actuation cylinder arranged outside the friction clutch and whose actuating piston is connected, via a disengagement lever, to a release bearing mounted to move axially on a guide sleeve fixed on the housing, which can be in contact with the spring tabs of a contact pressure spring made as a membrane spring. Alternatively, however, the clutch actuator can also be made as a central release device in which an annular actuating piston of an annular cylindrical actuating cylinder, arranged co-axially with the input shaft of the drive transmission, is connected directly to the release bearing.
In the simplest case, the clutch actuator is made as a simply operating actuating cylinder with an actuating piston, a pressure space and a restoring spring. Such a clutch actuator can be actuated in a simple mariner by way of at least one proportional control valve, in particular a 3/2-way magnetic control valve, comprising a first input connected to a pressure line, a second input connected to a line, which is not pressurized, and an output connected to the pressure space of the clutch actuator. However, it is also feasible here to use at least two timing valves, in particular 2/2-way magnetic timing valves, in each case, with an input connected to the pressure line or the pressureless line and an output connected to the pressure space of the clutch actuator.
The generation of a given clutch torque, for example a relatively large starting torque or a relatively small creeping torque, has until now been done exclusively by path control, i.e., in accordance with a torque characteristic that sets the clutch torque of the friction clutch as a function of the actuation path or the release path of the clutch actuator or of a transmission element, such as a release lever or a release bearing. For this, the corresponding actuation path is determined by a path sensor and the actuating pressure acting in the clutch actuator is changed, i.e., starting from the closed condition increased and starting from the open condition reduced, until the desired value of the actuation path has been reached. During this, if a difference between the actual value of the actuation path and its desired value is detected, then the actuating pressure is increased or reduced, depending on the direction of the difference, until the difference has been eliminated to within a specified tolerance.
Now if there is a large difference between the actual and desired actuation path positions, as can be caused by sticking because of wear, dirt or poor lubrication on the sliding surfaces between the release bearing and the guide sleeve or in a linkage of the release lever, then this results in a large difference between the actuating pressure of the clutch actuator and the associated actuation position. Until now, i.e., in the case of purely path-dependent control, to eliminate this actuation path difference the nominal pressure associated with the desired actuation path position has been correspondingly set too high, which can lead disadvantageously to jerky bridging and so to overrunning of the actuation path difference.
From this standpoint, a starting process is to be regarded as particularly critical, since starting from the open condition and with the starting gear engaged the friction clutch is closed in the direction of the desired starting torque so the actuating pressure of the clutch actuator is correspondingly reduced. In such a case, when a negative actuation path difference with too large an actual position of the actuation or release path occurs, the actuating pressure of the clutch actuator is necessarily reduced to a value lower than the nominal pressure that would correspond to the desired nominal clutch torque. But when the actuation path difference occurring is bridged with jerking and the friction clutch is over-actuated, i.e., closed too far, then at least for a short time a larger clutch torque is transmitted. This can result either in undesired, jerky starting of the motor vehicle or undesired stalling of the drive engine.
Thus, the purpose of the invention is to propose a method for controlling an automated friction clutch of the type mentioned above, where more exact control of the friction clutch is possible and the disadvantages described earlier can be avoided.